Not sure of current cost everywhere - others can reply. But be careful that you compare like with like. Belize Dive Connection (at Sunbreeze Hotel) includes a new PADI manual that you keep and take away with you (a PADI requirement often breached - separate cost maybe US$40), GST (sales tax at 12.5%), park fees for course dives you do in our local marine park, and equipment for those dives. They charge US$445, there are no extras, and they'll do a good job to full international standards.
Another SP operation I'd strongly recommend is Island Divers, on the dock of the Holiday Hotel. They will also do a good job and also supply a manual & equipment. Their price (with no extras) is US$425.
There are of course other SP operations. I've been hearing good reports of Ecologic Divers but I have no first-hand experience of them. Perhaps the biggest local operation is Amigos del Mar.
On Caye Caulker there are several operations, but I'd recommend going with Belize Diving Services, who operate to high standards. They are now the only technical diving operation in the country - I closed mine before they "went tech" under new owners. I wouldn't go with any of the other operations there.
You do just have time to do the classroom work before you come down. That's usually by registering for "eLearning" at the PADI website (there is another route). Overall it works out more expensive, but it does mean less studying here when you'd rather be partying! Assuming you do it all here, reckon on three full days and two evenings till you're certified Open Water.
If you need advice on accommodation on Ambergris Caye come back here. I don't know the accommodation on Caye Caulker well enough to help, but BDS will be able to advise.
If you come to San Pedro please bring a bottle of real maple syrup!
